MC3425 Datasheet, PDF (6/12 Pages) ON Semiconductor – POWER SUPPLY SUPERVISORY/ OVER AND UNDERVOLTAGE PROTECTION CIRCUIT

MC3425
CIRCUIT DESCRIPTION
The MC3425 is a power supply supervisory circuit
containing all the necessary functions required to monitor
over and undervoltage fault conditions. The block diagram
is shown below in Figure 15. The Overvoltage (O.V.) and
Undervoltage (U.V.) Input Comparators are both
referenced to an internal 2.5 V regulator. The U.V. Input
Comparator has a feedback activated 12.5 µA current sink
(IH) which is used for programming the input hysteresis
voltage (VH). The source resistance feeding this input (RH)
determines the amount of hysteresis voltage by VH = IHRH
= 12.5 × 10–6 RH.
Separate Delay pins (O.V. DLY, U.V. DLY.) are provided for
each channel to independently delay the Drive and Indicator
outputs, thus providing greater input noise immunity. The two
Delay pins are essentially the outputs of the respective input
comparators, and provide a constant current source,
IDLY(source), of typically 200 µA when the noninverting input
voltage is greater than the inverting input level. A capacitor
connected from these Delay pins to ground, will establish a
predictable delay time (tDLY) for the Drive and Indicator
outputs. The Delay pins are internally connected to the
noninverting inputs of the O.V. and U.V. Output Comparators,
which are referenced to the internal 2.5 V regulator.
Therefore, delay time (tDLY) is based on the constant current
source, IDLY(source), charging the external delay capacitor
(CDLY) to 2.5 V.
tDLY = _{IVDrLeYf (CsDouLrYce=)}
2.5 CDLY
200 µA
= 12500 CDLY
Figure 5 provides CDLY values for a wide range of time
delays. The Delay pins are pulled low when the respective
input comparator’s noninverting input is less than the
inverting input. The sink current, IDLY(sink), capability of the
Delay pins is ≥ 1.8 mA and is much greater than the typical
200 µA source current, thus enabling a relatively fast delay
capacitor discharge time.
The Overvoltage Drive Output is a current–limited
emitter–follower capable of sourcing 300 mA at a turn–on
slew rate at 2.0 A/µs, ideal for driving “Crowbar” SCR’s. The
Undervoltage Indicator Output is an open–collector, NPN
transistor, capable of sinking 30 mA to provide sufficient drive
for LED’s, small relays or shut–down circuitry. These current
capabilities apply to both channels operating simultaneously,
providing device power dissipation limits are not exceeded.
The MC3425 has an internal 2.5 V bandgap reference
regulator with an accuracy of ± 4.0% for the basic device.
Figure 15. Representative Block Diagram
O.V.
Sense
3
+ _Input
Comparator
– O.V.
VCC
8
+
+
200µA
+
+Output
Comparator
– _O.V.
U.V.
Sense
4
IH
12.5µA
+ _Input
Comparator
– U.V.
Input Section
Note: All voltages and currents are nominal.
+
200µA
52
U.V. O.V.
DLY DLY
–
Output
Comparator
+ ^U.V.
+
2.5V
Reference
Regulator
O.V.
1
DRV
U.V.
6
IND
7
Gnd ^{Output Section}
6
MOTOROLA ANALOG IC DEVICE DATA